Method to control a road vehicle for the execution of a multiple downshift in a drivetrain provided with a servo-assisted transmission

ABSTRACT

A method to control a road vehicle for the execution of a multiple downshift in a drivetrain provided with a servo-assisted transmission; the control method comprises the steps of: detecting a condition of slowing down of the road vehicle and, simultaneously, detecting a driver&#39;s request for a multiple downshift; carrying out, in succession, a plurality of downshifts while the road vehicle is slowing down and in an autonomous manner regardless of further interventions of the driver; determining a duration of a shift time interval; and carrying out each downshift following a first downshift when said shift time interval has exactly elapsed since the previous downshift.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Italian patent applicationno. 102019000017507 filed on Sep. 30, 2019, the entire disclosure ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to a method to control a road vehicle for theexecution of a multiple downshift in a drivetrain provided with aservo-assisted transmission.

The invention finds advantageous application in a drivetrain providedwith a dual-clutch, servo-assisted transmission, to which explicitreference will be made in the description below without because of thisloosing in generality.

PRIOR ART

A drivetrain provided with a dual-clutch, servo-assisted transmissioncomprises a pair of primary shafts, which are coaxial to one another,are independent of one another and are inserted inside one another; twocoaxial clutches, each designed to connect a respective primary shaft toa drive shaft of an internal combustion engine; and at least onesecondary shaft, which transmits the motion to the drive wheels and canbe coupled to the primary shafts by means of respective gear trains,each defining a gear.

During a gear shift, the current gear couples the secondary shaft to aprimary shaft, while the following gear couples the secondary shaft tothe other primary shaft; as a consequence, the gear shift takes place bycrossing the two clutches, namely by opening the clutch associated withthe current gear and by simultaneously closing the clutch associatedwith the following gear.

There is a function (called “sequential downshift”) which allows driversgear down multiple times while braking in order to engage, at the end ofthe braking, the lowest gear possible (depending on the maximum speed ofrotation of the internal combustion engine) so as to have the maximumacceleration possible when starting again (which takes place, indeed, atthe end of the braking). This function known as “sequential downshift”is activated by drivers during a braking (namely, when the brake pedalis pressed) by pressing a downshift paddle shifter for a long time(namely, for an amount of time exceeding a predetermined timethreshold), said downshift paddle shifter being normally pressed for ashort instant in order to request one single downshift (namely, theengagement of a new gear, which is lower than the current gear).

In other words, the driver, in order to control the dual-clutch,servo-assisted transmission, can rely on an upshift paddle shifter,which is briefly pressed in order to request one single upshift (namely,the engagement of a new gear, which is higher than the current gear andcontiguous with the current gear), and a downshift paddle shifter, whichis briefly pressed in order to request one single downshift (namely, theengagement of a new gear, which is lower than the current gear and iscontiguous with the current gear). During a braking (obviously, aprolonged one), drivers, while pressing the brake pedal, cancontinuously press the downshift paddle shifter in order to activate thefunction called “sequential downshift”, which automatically gears downmultiple times while braking so that, at the end of the braking (namely,when the brake pedal is released), the lowest gear possible is engaged.

The function known as “sequential downshift” is typically used whenracing on a track, at the end of a straight stretch, when drivers choseto ask this function to handle the selection of the ideal gear whilethey concentrate on preparing for the next bend.

The function called “sequential downshift” currently entails setting aseries of thresholds for the rotation speed of the internal combustionengine and, hence, automatically shifting gear every time the rotationspeed of the internal combustion engine reaches a corresponding rotationspeed threshold. However, the software generating the series of rotationspeed thresholds for the internal combustion engine requires a long andcomplicated adjustment; furthermore, the operating mode of the functionknown as “sequential downshift” is usually not appreciated by drivers,who tend to find it “unnatural” (namely, contrary to drivers'expectations), even though effective.

Some examples of implementation of the function called “sequentialdownshift” are described in patent applications WO2013153309A1 andEP2921746A1.

DESCRIPTION OF THE INVENTION

The object of the invention is to provide a method to control a roadvehicle for the execution of a multiple downshift in a drivetrainprovided with a servo-assisted transmission, said method not sufferingfrom the drawbacks discussed above and, at the same time, being easy andeconomic to be implemented.

According to the invention there is provided a method to control a roadvehicle for the execution of a multiple downshift in a drivetrainprovided with a servo-assisted transmission, according to the appendedclaims.

The appended claims describe preferred embodiments of the invention andform an integral part of the description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings, showing a non-limiting embodiment thereof, wherein:

FIG. 1 is a schematic plan view of a rear-wheel drive road vehicleprovided with a drivetrain with a dual-clutch, servo-assistedtransmission, which is controlled according to the control method of theinvention;

FIG. 2 is a schematic view of the drivetrain of FIG. 1; and

FIG. 3 shows the time development of the rotation speed of the twoclutches and of the drive shaft during the execution of a multipledownshift.

PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1, number 1 indicates, as a whole, a road vehicle (inparticular, a car) provided with two front driven (namely, non-drive)wheels 2 and with two rear drive wheels 3. In a front position there isan internal combustion engine 4, which is provided with a drive shaft 5,which produces a torque, which is transmitted to the drive wheels 3 bymeans of a drivetrain 6. The drivetrain 6 comprises a dual-clutch,servo-assisted transmission 7 arranged in the rear-wheel-drive assemblyand a transmission shaft 8, which connects the drive shaft 5 to an inputof the dual-clutch, servo-assisted transmission 7. The dual-clutch,servo-assisted transmission 7 is connected, in a train-like manner, to aself-locking differential 9, from which a pair of axle shafts 10 start,each integral to a drive wheel 3.

The road vehicle 1 comprises a control unit 11 of the engine 4, whichcontrols the engine 4, a control unit 12 of the drivetrain 6, whichcontrols the drivetrain 6, and a BUS line 13, which is manufactured, forexample, according to the CAN (Car Area Network) protocol, extends tothe entire road vehicle 1 and allows the two control units 11 and 12 tocommunicate with one another. In other words, the control unit 11 of theengine 4 and the control unit 12 of the drivetrain 6 are connected tothe BUS line 13 and, therefore, can communicate with one another bymeans of messages sent through the BUS line 13. Furthermore, the controlunit 11 of the engine 4 and the control unit 12 of the drivetrain 6 canbe directly connected to one another by means of a dedicatedsynchronization cable 14, which is capable of directly transmitting asignal from the control unit 12 of the drivetrain 6 to the control unit11 of the engine 4 without the delays caused by the BUS line 13.Alternatively, the synchronization cable 14 could be absent and allcommunications between the two control units 11 and 12 could beexchanged using the BUS line 13.

According to FIG. 2, the dual-clutch, servo-assisted transmission 7comprises a pair of primary shafts 15, which are coaxial to one another,independent of one another and inserted inside one another. Furthermore,the dual-clutch, servo-assisted transmission 7 comprises two coaxialclutches 16, each designed to connect a respective primary shaft 15 tothe drive shaft 5 of the internal combustion engine 4 through theinterposition of the transmission shaft 8; each clutch 16 is an oil bathclutch and, hence, is pressure-controlled (i.e. the degree ofopening/closing of the clutch 16 is determined by the pressure of theoil inside the clutch 16); according to an alternative embodiment, eachclutch 16 is a dry clutch and, hence, is position-controlled (i.e. thedegree of opening/closing of the clutch 16 is determined by the positionof a movable element of the clutch 16). The dual-clutch, servo-assistedtransmission 7 comprises one single secondary shaft 17 connected to thedifferential 9 that transmits the motion to the drive wheels 3;according to an alternative and equivalent embodiment, the dual-clutch,servo-assisted transmission 7 comprises two secondary shafts 17, bothconnected to the differential 9.

The dual-clutch, servo-assisted transmission 7 has seven forward gearsindicated with Roman numerals (first gear I, second gear II, third gearIII, fourth gear IV, fifth gear V, sixth gear VI and seventh gear VII)and a reverse gear (indicated with R). The primary shaft 15 and thesecondary shaft 17 are mechanically coupled to one another by aplurality of gear trains, each defining a respective gear and comprisinga primary gear wheel 18 fitted on the primary shaft 15 and a secondarygear wheel 19 fitted on the secondary shaft 17. In order to allow for acorrect operation of the dual-clutch, servo-assisted transmission 7, allodd gears (first gear I, third gear III, fifth gear V, seventh gear VII)are coupled to a same primary shaft 15, whereas all even gears (secondgear II, fourth gear IV and sixth gear VI) are coupled to the otherprimary shaft 15.

Each primary gear wheel 18 is splined to a respective primary shaft 15,so as to always rotate with the primary shaft 15 in an integral manner,and permanently meshes with the respective secondary gear wheel 19; onthe other hand, each secondary gear wheel 19 is mounted on the secondaryshaft 17 in an idle manner. Furthermore, the dual-clutch, servo-assistedtransmission 7 comprises four synchronizers 20, each mounted coaxial tothe secondary shaft 17, arranged between two secondary gear wheels 19and designed to be operated so as to alternatively fit the tworespective secondary gear wheels 19 to the secondary shaft 17 (i.e. soas to alternatively cause the two respective secondary gear wheels 19 tobecome angularly integral to the secondary shaft 17). In other words,each synchronizer 20 can be moved in one direction to fit a secondarygear wheel 19 to the secondary shaft 17 or can be moved in the otherdirection to fit the other secondary gear wheel 19 to the secondaryshaft 17.

The dual-clutch transmission 7 comprises one single secondary shaft 17connected to the differential 9 that transmits the motion to the drivewheels 3; according to an alternative and equivalent embodiment, thedual-clutch transmission 7 comprises two secondary shafts 17, bothconnected to the differential 9.

According to FIG. 1, the road vehicle 1 comprises a passengercompartment housing a driving position for the driver; the drivingposition comprises a seat (which is not shown), a steering wheel 21, anaccelerator pedal 22, a brake pedal 23 and two paddle shifters 24 and25, which control the dual-clutch, servo-assisted transmission 7 and areconnected to the opposite sides of the steering wheel 21. The upshiftpaddle shifter 24 is operated by the driver (by means of a shortpressure) in order to request an upshift (namely, the engagement of anew gear, which is higher than the current gear and contiguous with thecurrent gear), whereas the downshift paddle shifter 25 is operated bythe driver (by means of short pressure) in order to request a downshift(namely, the engagement of a new gear, which is lower than the currentgear and is contiguous with the current gear).

When the road vehicle 1 is slowing down (due to the driver acting uponthe brake pedal 23) and the driver holds the downshift paddle shifter 25pressed for a long time (namely, for an amount of time exceeding apredetermined time threshold), said downshift paddle shifter beingnormally pressed for a short instant in order to request one singledownshift, the control unit 12 of the drivetrain 6 activates a function(called “sequential downshift”) which allows drivers gear down multipletimes while braking in order to engage, at the end of the braking, thelowest gear possible (depending on the maximum speed of rotation of theinternal combustion engine 4) so as to have the maximum accelerationpossible when starting again (which takes place, indeed, at the end ofthe braking). This function known as “sequential downshift” determinesthe automatic execution (namely, with no intervention of the driver), insuccession, of a plurality of downshifts up to when the slowing down ofthe road vehicle 1 is interrupted (namely, up to when the driver stopspressing the brake pedal 23).

In other words, the driver, in order to control the dual-clutch,servo-assisted transmission 7, can rely on an upshift paddle shifter 24,which is briefly pressed in order to request one single upshift (namely,the engagement of a new gear, which is higher than the current gear andcontiguous with the current gear), and a downshift paddle shifter 25,which is briefly pressed in order to request one single downshift(namely, the engagement of a new gear, which is lower than the currentgear and is contiguous with the current gear). During a braking(obviously, a prolonged one), drivers, while pressing the brake pedal23, can continuously press the downshift paddle shifter 25 in order toactivate the function called “sequential downshift”, which automaticallygears down multiple times while braking so that, at the end of thebraking (namely, when the brake pedal 23 is released), the lowest gearpossible is engaged.

The function known as “sequential downshift” is typically used whenracing on a track, at the end of a straight stretch, when drivers choseto ask this function to handle the selection of the ideal gear whilethey concentrate on preparing for the next bend.

The control unit 12 of the drivetrain 6 detects a condition of slowingdown of the road vehicle 1 (by determining a degree of pressing of thebrake pedal 23, for example by detecting the pressure of the brake fluidin the hydraulic circuit of the braking system) and, simultaneously,detects a driver's request for a multiple downshift (if the driver holdsthe downshift paddle shifter 25 pressed for a long time); hence, if bothconditions discussed above occur, the control unit 12 of the drivetrain6 carries out, in succession, a plurality of downshifts while the roadvehicle 1 is slowing down and in an autonomous manner regardless offurther interventions of the driver. In particular, the control unit 12of the drivetrain 6 determines a duration of one single shift timeinterval Δt, which is common to all the plurality of downshifts (namely,said single shift time interval Δt applies in the same way to all thedownshifts of the plurality of downshifts), and carries out eachdownshift following a first downshift when said shift time interval Δthas exactly elapsed since the previous downshift.

According to a preferred embodiment, the control unit 12 of thedrivetrain 6 carries out the first downshift in a same instant T₁ (shownin FIG. 3) in which the driver's request for a multiple downshift isdetected and carries out all the other downshifts (following the firstdownshift) each time the shift time interval Δt has exactly elapsedsince the previous downshift; in this way, all downshifts are equallyspaced apart from one another, in terms of time, by the same shift timeinterval Δt (which is always constant, namely always the same). Namely,the duration of the shift time interval Δt always remains constant forall the downshifts carried out one after the other.

Obviously, the control unit 12 of the drivetrain 6 determines theplurality of downshifts when the slowing down condition of the roadvehicle 1 ceases, namely when the driver releases the brake pedal 23,thus ending the braking.

It should be pointed out that the control unit 12 of the drivetrain 6delays a downshift in case the latter is likely to cause an excessiveincrease in the rotation speed COE of the internal combustion engine 4(i.e. if it is likely cause the rotation speed ω_(E) of the internalcombustion engine 4 to exceed the maximum speed admitted); namely, ifneeded, a downshift is delayed until the downshift can be carried outwithout causing an excessive increase in the rotation speed ω_(E) of theinternal combustion engine 4.

According to a preferred embodiment, the control unit 12 of thedrivetrain 6 determines the duration of the shift time interval Δt basedon the rotation speed ω_(E) of the internal combustion engine 4 in theinstant T₁ in which the driver's request for a multiple downshift isdetected, based on a gear engaged in the dual-clutch, servo-assistedtransmission 7 in the instant T₁ in which the driver's request for amultiple downshift is detected, and/or based on a degree of pressing ofthe brake pedal 23 in the instant T₁ in which the driver's request for amultiple downshift is detected.

According to a possible embodiment, the control unit 12 of thedrivetrain 6 could have, in a memory of its, a map that provides theduration of the shift time interval Δt based on the rotation speed ω_(E)of the internal combustion engine 4, on the gear engaged in thedual-clutch, servo-assisted transmission 7 and on the degree of pressingof the brake pedal 23.

By way of example, the duration of the shift time interval Δt could beof 3-4 tenths of second in case of a high degree of pressing of thebrake pedal 23 and of 10-12 tenth of seconds in case of a low degree ofpressing of the brake pedal 23.

According to a preferred, though non-binding embodiment, the controlunit 12 of the drivetrain 6 determines a value of a comfort index (knownas “CMFidx”) in the instant T₁ in which the driver's request for amultiple downshift is detected and, hence, the control unit 12 of thedrivetrain 6 adjusts a speed of execution of the downshifts based on thevalue of the comfort index; namely, if the comfort index indicates thatthe driver cares about comfort, the downshifts are carried out moreslowly (smoothly) and, hence, trigger smaller longitudinal oscillations,whereas, if the comfort index indicates that the driver does not careabout comfort, the downshifts are carried out more quickly (suddenly)and, hence, trigger greater longitudinal oscillations. For example, thecontrol unit 12 of the drivetrain 6 could determine the value of thecomfort index based on the position of a selector (also known as “handlever”) controlled by the driver to indicate the type of driving styledesired.

FIG. 3 shows an example of a multiple downshift during a (prolonged)braking:

in the instant T₁, the driver (who is already pressing the brake pedal23) presses the downshift paddle shifter 25 and keeps pressing it for along time; therefore, in the instant T₁, the control unit 12 of thedrivetrain 6 carries out the first downshift (it should be pointed outthat, starting from the instant T₁, the rotation speed ω_(E) of theinternal combustion engine 4 changes from the rotation speed ω_(B) ofthe clutch 16B to the rotation speed ω_(A) of the clutch 16A) andcalculates the duration of the shift time interval Δt;

in the instant T₂ (which is exactly separated from the instant T₁ by theshift time interval Δt), the control unit 12 of the drivetrain 6autonomously carries out (namely, without further commands of thedriver) the second downshift (it should be pointed out that, startingfrom the instant T₂, the rotation speed ω_(E) of the internal combustionengine 4 changes from the rotation speed ω_(A) of the clutch 16A to therotation speed ω_(B) of the clutch 16B);

in the instant T₃ (which is exactly separated from the instant T₂ by theshift time interval Δt), the control unit 12 of the drivetrain 6autonomously carries out (namely, without further commands of thedriver) the third downshift (it should be pointed out that, startingfrom the instant T₃, the rotation speed ω_(E) of the internal combustionengine 4 changes from the rotation speed ω_(B) of the clutch 16B to therotation speed ω_(A) of the clutch 16A);

in the instant T₄ (which is exactly separated from the instant T₃ by theshift time interval Δt), the control unit 12 of the drivetrain 6autonomously carries out (namely, without further commands of thedriver) the fourth downshift (it should be pointed out that, startingfrom the instant T₄, the rotation speed ω_(B) of the internal combustionengine 4 changes from the rotation speed ω_(A) of the clutch 16A to therotation speed ω_(B) of the clutch 16B);

in the instant T₅ (which is separated from the instant T₄ by less thanthe shift time interval Δt, hence too soon for a downshift, which,indeed, does not take place), the driver releases the brake pedal 23(namely, ends the braking phase) and, therefore, the control unit 12 ofthe drivetrain 6 ends the plurality of downshifts, since the slowingdown condition of the road vehicle 1 ceases.

What disclosed above can be applied, with no significant changes, evenwhen the drivetrain 6 of the road vehicle 1 is provided with asingle-clutch, servo-assisted transmission.

The embodiments described herein can be combined with one another,without for this reason going beyond the scope of protection of theinvention.

The control method described above has different advantages.

First of all, the control method described above allows the adjustmentphase to be reduced and simplified to a significant extent, since thelaw that provides the duration of the shift time interval Δt can bedetermined and set up in relatively simple and quick manner.

Furthermore, the control method described above controls thedual-clutch, servo-assisted transmission 7 in ways that are generallyappreciated by drivers, who deem them to be “natural” (namely,corresponding to the drivers' expectations).

Finally, the control method described above is easy and economic to beimplemented as its execution requires a limited memory space and areduced calculation ability.

LIST OF THE REFERENCE NUMBERS OF THE FIGURES

-   1 road vehicle-   2 front wheels-   3 rear wheels-   4 engine-   5 drive shaft-   6 drivetrain-   7 transmission-   8 transmission shaft-   9 differential-   10 axle shafts-   11 engine control unit-   12 drivetrain control unit-   13 BUS line-   14 synchronization cable-   15 primary shafts-   16 clutches-   17 secondary shaft-   18 primary gear wheel-   19 secondary gear wheel-   20 synchronizers-   21 steering wheel-   22 accelerator pedal-   23 brake pedal-   24 upshift paddle shifter-   25 downshift paddle shifter-   ω_(E) rotation speed-   ω_(A) rotation speed-   ω_(B) rotation speed-   T₁ time instant-   T₂ time instant-   T₃ time instant-   T₄ time instant-   T₅ time instant-   Δt time interval

1) A method to control a road vehicle (1) for the execution of amultiple downshift in a drivetrain provided with a servo-assistedtransmission (7); the control method comprises the steps of: detecting acondition of slowing down of the road vehicle (1) and, simultaneously,detecting a driver's request for a multiple downshift; carrying out, insuccession, a plurality of downshifts while the road vehicle (1) isslowing down and in an autonomous manner regardless of furtherinterventions of the driver; determining a duration of a single shifttime interval (Δt) common to all the plurality of downshifts; andcarrying out each downshift following a first downshift when said shifttime interval (Δt) has elapsed exactly since the previous downshift. 2)The control method according to claim 1, wherein the first downshift iscarried out in a same instant (T₁) in which the driver's request for amultiple downshift is detected. 3) The control method according to claim1, wherein a downshift is delayed if it would cause an excessiveincrease in a rotation speed (ω_(e)) of an internal combustion engine(4) connected to the drivetrain (6). 4) The control method according toclaim 1, wherein the duration of the shift time interval (Δt) isdetermined based on a rotation speed (ω_(e)) of an internal combustionengine (4) in an instant (T₁) in which the driver's request for amultiple downshift is detected. 5) The control method according to claim1, wherein the duration of the shift time interval (Δt) is determinedbased on a gear engaged in the servo-assisted transmission (7) in aninstant (T₁) in which the driver's request for a multiple downshift isdetected. 6) The control method according to claim 1, wherein theduration of the shift time interval (Δt) is determined based on a degreeof pressing of a brake pedal (23) in an instant (T₁) in which thedriver's request for a multiple downshift is detected. 7) The controlmethod according to claim 1, wherein the duration of the shift timeinterval (Δt) always remains constant for all downshifts. 8) The controlmethod according to claim 1, wherein the plurality of downshifts endswhen the condition of slowing down of the road vehicle (1) ceases. 9)The control method according to claim 1, wherein the condition ofslowing down of the road vehicle (1) is detected by determining a degreeof pressing of a brake pedal (23). 10) The control method according toclaim 1, wherein the driver's request for a multiple downshift isdetected if the drivers keeps pressing a down paddle shifter (25) for along time. 11) The control method according to claim 1 and comprisingthe further steps of: determining a value of a comfort index in aninstant (T₁) in which the driver's request for a multiple downshift isdetected; and adjusting a speed of execution of the downshifts based onthe comfort index.